(CNN) -- The Tuskegee syphilis experiment of the 20th century is often cited as the most famous example of unethical medical research. Now, evidence has emerged that it overlapped with a shorter study, also sponsored by U.S. government health agencies, in which human subjects were unknowingly being harmed by participating in an experiment.

Research from Wellesley College professor Susan Reverby has uncovered evidence of an experiment in Guatemala that infected people with sexually transmitted diseases in an effort to explore treatments.

The U.S. government apologized for the research project on Friday, more than 60 years after the experiments ended. Officials said an investigation will be launched into the matter.

[...]

"In a racist context, they thought [syphilis] might be different in African-Americans; the real unethical part in my mind had to do with denial of treatment and, most importantly, the denial of information about the study to the men involved," he said.